Magnetic separation and purification processes using magnetic particles as a solid extraction phase are widely used e.g. in clinical chemistry assays for medical diagnostic purposes, wherein target molecules or target particles are bound on suitable magnetic particles and labeled with a specific receptor, and these method steps are followed by a step wherein the magnetic particles carrying target particles bound on them are separated from the liquid where they were originally suspended by means of a high magnetic field gradient.
Within the scope of this description the terms target molecules or particles are used to designate in particular any biological components such as cells, cell components, bacteria, viruses, toxins, nucleic acids, hormones, proteins and any other complex molecules or the combination of thereof.
The magnetic particles used are e.g. paramagnetic or superparamagnetic particles with dimension ranging from nanometric to micrometric scales, for instance magnetic particles of the types mentioned in the publication of B. Sinclair, “To bead or not to bead,” The Scientist, 12[13]:16-9, Jun. 22, 1998.
The term specific receptor is used herein to designate any substance which permits to realize a specific binding affinity for a given target molecule, for instance the antibody-antigen affinity (see e.g. U.S. Pat. No. 4,233,169) or glass affinity to nucleic acids in a salt medium (see e.g. U.S. Pat. No. 6,255,477.
Several systems using magnetic separation and purification process have been developed during the two last decades and have led to a large variety of commercially available apparatus which are miniaturized and automated to some extent, but there has been relatively little progress in the development of the means used in those apparatuses for handling the magnetic particles. Basically the process comprises the step of mixing of a liquid sample containing the target molecules or particles with magnetic particles within a reservoir in order that the binding reaction takes place and this step is followed by a separation step of the complexes magnetic particle/target particle from the liquid by means of a permanent magnet or an electromagnet. Since this separation step is usually carried out with the liquid at rest, this step is known as static separation process. In some systems additional steps required for handling of the liquids involved (liquid sample, liquid reagent, liquid sample-reagent mixtures) are carried out by pipetting means.
A flow-through system for carrying out the separation of the magnetic particles, a so called dynamic separation system, is more advantageous than a static separation system, in particular because it makes possible to effect separation of magnetic particles and steps involving liquid processing with more simple means and with more flexibility.
However, only few magnetic separation systems are known and they have serious drawbacks. In most of them the magnetic particles retained build a cluster deposited on the inner wall of a flow-through cell and for this reason the perfusion of the target molecules is inefficient.
According to U.S. Pat. No. 6,159,378 this drawback can be partially overcome by inserting in the flow path of the liquid carrying the target molecules or target particles a filter structure made magnetic flux conducting material, and by applying a magnetic field to that filter structure. A serious drawback of this approach is that the filter structure is a source of contamination or cross-contamination problems.